The invention described herein arose in the course of, or under, Contract No. DE-AC03-76SF00098 between the United States Department of Energy and the University of California.
The invention relates to cathodes for electron discharge devices and more particularly to a field free, directly heated lanthanum boride cathode useful in a device having positive ion flow.
Ehlers et al, in an article entitled "Characteristics of the Berkeley Multicusp Ion Source", Rev. Sci. Instrum. 50 (11), November 1979, at pp. 1353-1361, discuss the short lifetimes of tungsten filaments operated with a long pulse duration and suggest the use of lanthanum hexaboride or impregnated oxide cathodes instead. The impregnated oxide cathode is described as a cylindrical cathode mounted on a molybdenum holder and heated internally by a noninductive tungsten filament.
The use of lanthanum boride as a cathode material is attractive because of its high melting point, chemical inertness, low work function, and resistance to erosion under ion bombardment When heated to a temperature of 1600.degree. K. or higher, lanthanum boride is a copious emitter of electrons.
Leung et al, in an article entitled "Directly Heated Lanthanum Hexaboride Filaments", published in Rev. Sci. Instrum. 55 (7), July 1984, at pp. 1064-1068, describes physical properties of lanthanum hexaboride filaments, when operated as cathodes in a gas discharge. The use of a hairpin like configuration for the lanthanum hexaboride filament is described to compensate for thermal expansion of the material.
Pincosy et al, in "Lanthanum Hexaboride Tapered Filament in a Plasma Source", Rev. Sci. Instrum. 56 (5), May 1985, at pp. 655-658, discuss results obtained using a lanthanum hexaboride filament in a plasma source wherein the width of the filament is tapered to obtain a more uniform temperature distribution along the filament to provide a uniform electron emission along the entire length of the filament.
Goebel et al in "Large-Area Lanthanum Hexaboride Electron Emitter", Rev. Sci. Instrum. 56 (9), September 1985, at pp. 1717-1722, discuss the characteristics of lanthanum-boron thermionic emitters and describe a large-area, indirectly heated, continuously operated lanthanum boride cathode assembly and the tungsten filament heater used to indirectly heat the lanthanum boride.
The use of lanthanum hexaboride material as a cathode for thermionic emission of electrons is also described in Brunger et al U.S. Pat. Nos. 4,258,283 and Clerc 4,429,250. Goebel et al. 4,297,615 describes the use of a lanthanum hexaboride cylindrical cathode in a plasma generating device. The lanthanum hexaboride cathode is indirectly heated by a non-inductive tungsten heater supported in the cathode cylinder by support rods.
Morimiya et al U.S. Pat. No. 4,339,691 discloses a discharge apparatus containing a hollow cylindrical cathode made from a refractory metal and coated on the inside with an electron emitter such as lanthanum hexaboride. A heater is placed within the cylindrical cathode which is insulated from the cathode. The hollow cathode is initially heated by the heater after which a source of ionizing gas is fed into the hollow cathode and a discharge arc is ignited between the heater and the cathode. Plasma obtained within the hollow cathode is said to drift through an opening in the cathode toward the anode within the discharge apparatus to establish an arc between the cathode and the anode. In another embodiment, the heater is replaced by a disk electrode located within the hollow cathode which is made more positive than the cathode to cause the cathode to emit electrons. Hydrogen gas is then admitted to the hollow cathode and a plasma is established between the disk electrode and the cathode which causes both to be heated by ion bombardment. This plasma then drifts toward the anode and sets up an arc between the cathode and the anode as before.
When a cathode used in a plasma generating device is directly heated to emit electrons, nonuniform emission of electrons from the cathode can result from nonuniform temperature distribution along the cathode unless the cathode is specially shaped to provide the necessary uniform temperature distribution.
Even if such uniform temperature is achieved through the use of a specially designed directly heated cathode, the magnetic fields generated by the current flowing through the cathode to heat it can be strong enough to interfere with electron emission from the cathode when the discharge voltage is low, i.e., less than about 70 volts.
It would, therefore, be desirable to have a directly heated lanthanum boride cathode in which the magnetic field generated by the heater current would be minimized.